Diatoms are microscopic unicellular aquatic organisms that have a siliceous cell wall (50 nm thick). They have different shapes and sizes varying between 1 and 10 microns and the diatoms are constituted of nano-pores, self-organized nano-channels, which gives them a large specific surface area of the order of 30 to 200 m2/g. This large specific surface area offers the possibility of making a high density of carbon nanotubes (CNTs) as well as having a large contact surface with the matrix.
Diatomites are defined as siliceous rock of organic origin.
Producing a biphasic CNT/diatom silica material is a technological challenge to take up in so far as diatoms are structures with complex shapes and because growing carbon nanotubes requires several steps according to the growth method chosen.
It is supposed that using CNTs to replace carbon black (amorphous carbon) in composite materials will boost the emergence of new materials with all sorts of new interesting properties.
Indeed, CNTs are known to have better properties than carbon black. Improved wear resistance and/or better retention of mechanical properties may be cited, among others.
Methods for chemical deposit in the vapour phase allow the deposit of films or nanoparticles onto substrates from precursors injected near the substrate in the gas phase. The principle consists in putting the precursors, which may be liquid, solid or gas, in a gaseous form and making them react together to form a film or solid nanostructures on the substrate. The advantage of gas phase methods compared to liquid phase methods is being able to consider all types of substrates, including substrates having complex morphology, such as diatoms.
Atomic layer deposition (ALD) (see FIG. 1) consists in alternatively reacting the precursors at the surfaces of the substrates and presents the advantage of being the technique that leads to the highest coverage (He M., et al., Nano Res., 2010, 4, 334-342).
The principle of ALD consists in alternatively injecting two precursors separated by a purge gas. The precursors may be solid, liquid or gas but they are transformed into vapour before being sent to the reaction chamber. The steps of the process for an ALD cycle are described as follows:                a. Introduction of precursor 1: regardless of its nature (solid, liquid or gas), this must be vaporized from its container and transported by an inert carrier gas to the reaction chamber. This precursor adsorbs on the surface of the substrate. The duration of exposure of the precursor is a parameter that influences the structure of the film since it is what modulates how the molecules saturate the surface of the precursors.        b. Purge: after precursor 1 is adsorbed onto the substrate, an inert gas is added to purge and evacuates the molecules that could not be adsorbed onto the surface to the pump.        c. Introduction of precursor 2: the vapour from precursor 2 (or reactant) is introduced into the chamber and is intended to react with the molecules of precursor 1 that are already adsorbed. The reaction product forms the first atomic layer of the film. This type of precursor is often a reducing agent, an oxidizing agent, a nitrosing agent, an agent that can remove a ligand from precursor 1 or provide a missing element.        d. Second purge: the chamber is purged again to evacuate all the by-products of the reaction between the two precursors.        
The temperature, pressure, and number of cycles also influence the ALD technique.
A second method that is used to increase the carbon nanotubes is the method called chemical vapour deposition (CVD). The substrate is exposed to one or more precursors in the gas phase, which react and/or decompose on the surface of the substrate to generate the desired deposit.
A third method that may be envisaged is the vapour-impregnation method, which consists in saturating the surface of the substrate by the molecules intended to be grafted. In reality this method is a variant of CVD that is used when the substrate that has to be functionalized is fibrous in nature.
The formation of vertical carbon nanotubes has been reported (Duraia E. M., et al., Vacuum, 2010, 84, 464-468) (CNT) using diatomite as substrate, therefore combining the advantages of CNT (electric and optical properties) and those of diatomite (natural inexpensive material that is useful in water purification) in a single material. The technique of chemical vapour deposition assisted by microwave plasma was used for CNT growth on diatomite. A pre-treatment step was necessary before the growth step for CNTs whose main source of carbon is methane (CH4).